The above-referenced patent describes and claims interface apparatus for enabling conventional automotive diagnostic and test equipment to be used with certain types of direct ignition systems, also referred to as wasted spark or distributorless ignition systems. In a wasted spark ignition system, an individual ignition coil secondary winding is provided for each pair of cylinders, with the ends of the winding being connected to the spark plugs of the respective cylinders. A six cylinder engine, therefore, has six spark plugs and three secondary coil windings. There is no ground voltage reference. When an individual ignition coil secondary winding is "fired," opposite polarity voltages are developed across its ends, and the two spark plugs connected to the winding are fired together. The cylinder pairs are arranged so that one spark plug fires near the end of the compression stroke of its cylinder and its opposite paired spark plug fires near the end of the exhaust stroke of its cylinder. In practice, the wasted spark, that is, the spark produced near the end of the exhaust stroke of its cylinder, has a smaller amplitude than its counterpart power spark because it is not exposed to a gasoline/air mixture that is under compression. The wasted spark actually occurs earlier since the breakdown strength of the exhaust mixture is significantly lower than the breakdown strength of the charged (pressurized) gasoline/air mixture. Since both spark plugs are connected in series through the ignition coil secondary winding, the wasted spark circuit is initially completed through the parasitic capacitances of the system, that is, the capacitances of the spark plug, spark plug wires, etc. Upon occurrence of the power spark a few nanoseconds later, its conductive path includes the parasitic capacitances in addition to the discharge path through the other spark plug.
Most currently available engine tune-up equipment use a dedicated pickup and lead to sense the firing of the #1 cylinder in order to perform engine timing measurements and to synchronize the per cylinder information to specific cylinders in the firing order. In a conventional engine, there is only one cylinder firing event for each complete engine cycle (two engine revolutions in a four-cycle engine). Consequently, there is no confusion when the #1 pickup lead connected to a tester develops a signal. In a wasted spark system, on the other hand, the #1 pickup lead will experience two signals--one for the power firing of the #1 cylinder and another during the power firing of its complementary pair cylinder. On some distributorless ignition systems, there are signals that interface between the Electronic Control Module (ECM) and the distributorless ignition module and which are accessible for detection by engine test equipment to identify the occurrence of a cylinder #1 firing event. As used herein, a #1 signal will mean a signal that identifies a cylinder #1 firing event, irrespective of its origin in the engine. Thus, a tester for such engines must somehow be able to identify the power signal (from the wasted signal) since all of its internal timing and synchronization of data is dependent upon that.
In the disclosed apparatus of the above-referenced patent, the cylinder #1 power firing event signal must, therefore, be differentiated from the cylinder #1 wasted firing event signal so that proper synchronizing information may be applied to the tester. A power firing event is herein defined as that corresponding to the cylinder being spark ignited near the end of its compression stroke (or near the beginning of its power stroke) and a wasted firing event is that which occurs when the cylinder is fired near the end of its exhaust stroke (or near the beginning of its intake stroke). Since the power firing event signals are larger than the wasted firing event signals, they can generally be differentiated based upon amplitude. In the application, the signals are compared with a fixed reference to determine which are the power firing event signals.
There are inherent deficiencies in such a system because the signal amplitudes are dependent upon spark plug condition and the type of vehicle. Also, since the wasted spark path and the power spark path are substantially coextensive and are in series, a problem in either of the two cylinders, or in either of the two spark plugs or in the wiring will affect the signal waveforms captured. Therefore, the amplitudes of the two firing event signals may be insufficient to assure proper engine synchronization under a variety of operating conditions. The present invention solves that problem by "sorting" the two different types of firing event signals from all of the engine cylinders into two separate groups and obtains a weighted average of the relative amplitudes of the groups to determine the group that represents the power firing events and the group that represents the wasted firing events. In this way, one or two abnormally operating cylinders or spark plugs will not significantly affect the outcome of the determination. That information is then used, in conjunction with a cylinder #1 timing signal developed by a clamp-on inductive or capacitive pickup on the #1 spark plug wire (or by any other accessible #1 signal from the engine) and suitable logic, for determining which of the two firing events that occur for cylinder #1 is the power firing event.